1. Field of the Invention
This invention relates to an imaging optical system in which the state of aberration can be varied.
2. Description of the Prior Art
In an imaging optical system, it has heretofore been practiced to control aberration in order to improve the aberration or cause a predetermined aberration to occur as in a soft focus.
Generally, in an optical system comprised of a spherical lens group, a non-spherical surface is used as the surface of a part of the lens to correct aberration. However, the working of a non-spherical lens requires a high degree of accuracy using sophisticated techniques and accordingly is very expensive.
Further, some photographing lenses achieve the soft focus function by their entrance pupil being formed into a special shape. In such a method, however, if the diameter of the aperture diaphragm is varied, the soft focus functions correspondingly to the variation in the aperture diameter and therefore, such a method is inconvenient to photographers who require a degree of freedom of the soft focus function.